1. Field of the Invention
This invention relates to a method of processing a thermal cracked by-product oil containing aromatic hydrocarbon compounds from high-temperature cracking of petroleum hydrocarbons in the presence of an acid catalyst to achieve improvement in the composition of the distillates from the thermal cracked by-product oil, reduction of unsaturated components and production of useful heavy products.
2. Description of the Prior Art
So-called naphtha cracking which represents pyrolysis of various petroleum hydrocarbons such as naphtha at a high temperature of 700.degree. C. or higher to produce basic materials for petrochemical industries such as ethylene and propylene is widely employed. Depending upon the nature of the starting oil to be fed to the cracking apparatus, there are produced 0.5-3.0 parts by weight of by-product oil per one part by weight of ethylene product. A distillate of the by-product oil containing components of a boiling range between 75.degree. C. and 198.degree. C. is so-called cracked gasoline distillate which contains as the principal component aromatic hydrocarbons and from which are recovered or produced via an aromatics extraction step, benzene, toluene, mixed-xylenes, C.sub.9 -alkyl benzenes, C.sub.10 -alkyl benzenes, and the like.
However, as the distillate contains, in addition to the principal aromatic hydrocarbon component, aliphatic saturated hydrocarbons as well as unsaturated components which are aromatic olefins and a minor amount of other unsaturated hydrocarbons, there is included the removal of unsaturated components as a pretreatment step in the conventional aromatics extraction step in order to avoid adverse results such as clogging of the apparatus due to polymerization of the unsaturated components and deterioration due to contamination of the recovered aromatics.
On the other hand, catalytic reformed gasoline, which is an important aromatics source in petrochemical industries compared to thermal cracked by-product oil, has a very low content of unsaturated components (bromine number of 3.8 or below) and there is little need in the said reformed gasoline for a step of removing unsaturated components as a pretreatment step to the aromatics extraction step.
A method of removing unsaturated components of thermal cracked by-product oil which has been put into practical use on an industrial scale is the selective hydrogenation process. Selective hydrogenation requires complete processing of the unsaturated components while inhibiting conversion of the aromatic components to cyclohexanes and polymerization of styrenes. Thus, there are usually associated with this process difficulties in selecting reaction conditions. For example, there is employed at the first stage, hydrogenation of polymerizable components under moderate conditions in order to prevent polymerization of styrenes, followed by the second stage of complete hydrogenation to remove the unsaturated components. In addition, a large amount of hydrogen is consumed in the selective hydrogenation process. Moreover, the hydrogenation results in conversion of the unsaturated components only to substances of little economic value such as alkyl benzenes and paraffins so that resources useful in chemical industry are not efficiently utilized.
For example, styrene which is a main constituent of the unsaturated components in the aforementioned thermal cracked by-product oil, is converted by the hydrogenation into ethylbenzene. As a result, the ethylbenzene content in the xylene distillate obtained by the aromatics extraction step after hydrogenation processing is increased.
Heretofore, mixed-xylenes which have been obtained on an industrial scale have been separated and purified from aromatics-containing distillates of catalytic reformed oil, of naphtha, thermal cracked by-product oil naphtha, or the like, by such means as solvent extraction, extractive distillation, or the like.
The mixed-xylenes contain as principal components o-xylene, m-xylene, p-xylene, ethylbenzene, and the like, and their industrial application is generally as solvents or chemical raw materials. Whereas their use as solvents is not restricted being independent, upon the composition of xylene isomers therein, for their use as chemical raw materials the particular values of p-xylene and o-xylene in the isomers are important. p-Xylene is a starting material for the production of terephthalic acid which finds much demand as a raw material for the production of synthetic fibers. o-Xylene is a starting material for phthalic acid which is a starting material for plasticizers. On the contrary, m-xylene and ethylbenzene are of such little value that their values are increased by conversion of m-xylene by isomerization into p-xylene and conversion of ethylbenzene by isomerization into xylenes or by dehydrogenation into styrene. However, the isomerization of m-xylene is accompanied by high cost in the isomerization step and the ethylbenzene separated from mixed-xylenes is not economically suitable for the industrial production of styrene.
As shown in Table 1, components of mixed-xylenes are close in boiling point, and isolation of each component with only distillation is difficult.
Table 1 ______________________________________ Component Boiling point (.degree.C.) Melting point (.degree.C.) ______________________________________ Ethylbenzene 136.19 -94.98 p-xylene 138.35 +13.26 m-xylene 139.10 -48.87 o-xylene 144.41 -25.18 ______________________________________
The Industrial process for isolating each component of mixed-xylenes is usually a combination of rectification and a low temperature process. First, ethylbenzene and o-xylene which have different boiling points are separated by rectification. However, as it is impractical to separate m-xylene and p-xylene, these isomers being of narrower difference in boiling point, a low temperature process utilizing the difference in melting point is employed, wherein p-xylene crystallized is separated. In either of these separation steps, much energy is consumed so that mixed-xylenes with a higher content of p-xylene but a lower content of m-xylene and ethylbenzene are desirable.
The above-described considerations are important in particular in the case where aromatic hydrocarbon distillates, that is, cracked gasoline distillate, are used, which is distillated from a by-product of cracking of petroleum hydrocarbons such as naphtha conducted principally to obtain ethylene.
A variety of processes have been proposed in order to overcome the above-cited problems by means, for example, of removal of styrene, the precursor of ethylbenzene, in order to reduce ethylbenzene contained in the xylene distillate, at a stage prior to the hydrogenation process such as separation of styrene by rectification, extractive distillation, or selective adsorption with an absorbent. These processes, however, can hardly be considered the most advantageous because of the little value of the styrene separated thereby.